Phenomenology of summer ozone episodes over the Madrid Metropolitan Area, central Spain

https://doi.org/10.5194/acp-18-6511-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License.

Phenomenology of summer ozone episodes over the Madrid

Metropolitan Area, central Spain

Xavier Querol1, Andrés Alastuey1, Gotzon Gangoiti2, Noemí Perez1, Hong K. Lee3, Heeram R. Eun3, Yonghee Park3, Enrique Mantilla4, Miguel Escudero5, Gloria Titos1, Lucio Alonso2, Brice Temime-Roussel6, Nicolas Marchand6, Juan R. Moreta7, M. Arantxa Revuelta7, Pedro Salvador8, Begoña Artíñano8, Saúl García dos Santos9,

Mónica Anguas10, Alberto Notario11, Alfonso Saiz-Lopez10, Roy M. Harrison12,13, Millán Millán4, and Kang-Ho Ahn3

1_{Institute of Environmental Assessment and Water Research (IDAEA-CSIC), C/ Jordi Girona 18-26, Barcelona, 08034, Spain} 2_{Escuela Técnica Superior Ingeniería de Bilbao, Departamento Ingeniería Química y del Medio Ambiente, Universidad del}

País Vasco UPV/EHU, Urkixo Zumarkalea, S/N, Bilbao, 48013, Spain

3_{Department of Mechanical Engineering, Hanyang University, Ansan 425-791, Republic of Korea}

4_{Centro de Estudios Ambientales del Mediterráneo, CEAM, Unidad Asociada al CSIC, Parque Tecnológico C/ Charles R.}

Darwin, 14 Paterna, Valencia, 46980, Spain

5_{Centro Universitario de la Defensa de Zaragoza, Academia General Militar, Ctra. de Huesca s/n, Zaragoza, 50090, Spain} 6_{Aix Marseille Univ, CNRS, LCE, Marseille, France}

7_{Agencia Estatal de Meteorología, AEMET, C/ Leonardo Prieto Castro, 8, Madrid, 28071, Spain} 8_{Department of Environment, CIEMAT, Joint Research Unit Atmospheric Pollution CIEMAT-CSIC,}

c/ Avenida Complutense 40, Madrid, 28040, Spain

9_{Centro Nacional de Sanidad Ambiental – Instituto de Salud Carlos III (ISCIII), Ctr Majadahoda a Pozuelo km 2,}

Majadahonda (Madrid), 28222, Spain

10_{Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano,}

CSIC, Madrid, 28006, Spain

11_{University of Castilla-La Mancha, Physical Chemistry Department, Faculty of Chemical Science}

and Technologies, Ciudad Real, Spain

12_{National Centre for Atmospheric Science, University of Birmingham, B15 2TT, UK} 13_{Department of Environmental Sciences/Centre for Excellence in Environmental Studies,}

King Abdulaziz University, Jeddah, Saudi Arabia

Correspondence:Xavier Querol ([email protected]) Received: 30 October 2017 – Discussion started: 27 November 2017 Revised: 5 March 2018 – Accepted: 9 April 2018 – Published: 8 May 2018

Abstract.Various studies have reported that the photochem-ical nucleation of new ultrafine particles (UFPs) in urban en-vironments within high insolation regions occurs simultane-ously with high ground ozone (O3)levels. In this work, we evaluate the atmospheric dynamics leading to summer O3

episodes in the Madrid air basin (central Iberia) by means of measuring a 3-D distribution of concentrations for both pollutants. To this end, we obtained vertical profiles (up to 1200 m above ground level) using tethered balloons and miniaturised instrumentation at a suburban site located to the SW of the Madrid Metropolitan Area (MMA), the

Majada-honda site (MJDH), in July 2016. Simultaneously, measure-ments of an extensive number of air quality and meteorolog-ical parameters were carried out at three supersites across the MMA. Furthermore, data from O3soundings and daily radio

soundings were also used to interpret atmospheric dynamics. The results demonstrate the concatenation of venting and accumulation episodes, with relative lows (venting) and peaks (accumulation) in O3 surface levels. Regardless of

the episode type, the fumigation of high-altitude O3

are characterised by a relatively thinner planetary boundary layer (< 1500 m at midday, lower in altitude than the oro-graphic features), light synoptic winds, and the development of mountain breezes along the slopes of the Guadarrama Mountain Range (located W and NW of the MMA, with a maximum elevation of > 2400 m a.s.l.). This orographic– meteorological setting causes the vertical recirculation of air masses and enrichment of O3in the lower tropospheric

lay-ers. When the highly polluted urban plume from Madrid is affected by these dynamics, the highest Ox(O3+NO2)

con-centrations are recorded in the MMA.

Vertical O3profiles during venting episodes, with strong

synoptic winds and a deepening of the planetary boundary layer reaching > 2000 m a.s.l., were characterised by an up-ward gradient in O3levels, whereas a reverse situation with

O3 concentration maxima at lower levels was found during

the accumulation episodes due to local and/or regional pro-duction. The two contributions to O3surface levels

(fumiga-tion from high-altitude strata, a high O3background, and/or

regional production) require very different approaches for policy actions. In contrast to O3 vertical top-down transfer,

UFPs are formed in the planetary boundary layer (PBL) and are transferred upwards progressively with the increase in PBL growth.

1 Introduction

The EU Directive 2008/50/EC (2008) on ambient air quality, amended by Directive 2015/1480/EC (2015), establishes the need to comply with air quality standards to protect citizens and ecosystems. If these are not met, plans to improve air quality must be implemented by national, regional, and lo-cal administrations. Despite the considerable improvements in air quality during the last decade, non-compliance with European air quality standards is still reported in most of Europe. In particular, the limit values for nitrogen dioxide (NO2), particulate matter (PM10 and PM2.5), and the

tro-pospheric ozone (O3)target value are frequently exceeded

(EEA, 2017). Therefore, in 2013, the National Plan for Air Quality and Protection of the Atmosphere (Plan AIRE) 2013–2016 was drawn up and approved by the Spanish Council of Ministers’ Agreement of 12 April 2013.

The EEA (2017) recently reported that in 2015, 80 % of the urban EU-28 population was exposed to PM2.5levels

ex-ceeding the WHO guideline and 90 % to that of O3.

Measures to effectively reduce NO2and primary PM

pol-lution are relatively easy to identify (such as abating indus-trial, shipping, and traffic emissions with catalytic converters for NOXand particulate controls for PM). However, defining

policies for abating O3, other photochemical pollutants, and

the secondary components of PM is much more complex. Photochemical pollution is a subject of great environmen-tal importance in southern Europe due to its climatic and

geographical characteristics (Ochoa-Hueso, 2017). Products of this type of pollution are many, the most noteworthy be-ing tropospheric O3, secondary PM (nitrate, sulfate, and

sec-ondary organic compounds), and the generation of new ul-trafine particles (UFPs) by nucleation (Gomez-Moreno et al., 2011; Brines et al., 2015).

In summer, the Western Mediterranean Basin (WMB), surrounded by high mountains, falls under the influence of the semi-permanent Azores anticyclone. Clear skies prevail under a generalised level of subsidence aloft, and meso-meteorological processes with marked diurnal cycles dom-inate. Recirculation, strong insolation, and stability in the upper layers favour the production and accumulation of O3

(Millán et al., 1997, 2000, 2002; Kalabokas et al., 2008; Gi-annakopoulos et al., 2009; Velchev et al., 2011; Sicard et al., 2013) and the emissions of biogenic volatile organic com-pounds (BVOCs; Giannakopoulos et al., 2009).

The abatement of tropospheric O3levels in this region is

a difficult challenge due to its origin, which may be local, regional, and/or transboundary (Millán et al., 2000; Millán, 2014; Lelieveld et al., 2002; Kalabokas et al., 2008, 2013, 2015, 2017; Velchev et al., 2011; Sicard et al., 2013; Zanis et al., 2014), the complexity of the meteorological scenarios leading to severe episodes (Millán et al., 1997; Gangoiti et al., 2001; Dieguez et al., 2009, 2014; Kalabokas et al., 2017), and the complexity of the non-linear chemical processes that drive its formation and sinks (Monks et al., 2015, and refer-ences therein).

This complex context has led to a lack of “sufficient” O3

abatement in Spain (and Europe), while for primary pollu-tants, such as SO2and CO, and the primary fractions of PM10

and PM2.5improvement has been very evident (EEA, 2017).

Thus, the latest air quality assessment for Europe (EEA, 2017) shows the following: (i) there has been a tendency for the peak O3concentration values (those exceeding the hourly

information threshold of 180 µg m−3)to decrease in recent years, although not enough to meet the WHO guidelines and EC standards; and (ii) the problem of O3 episodes is more

pronounced in the south than in northern and central Europe. Likewise, O3 levels are higher in rural than in urban areas,

both due to (i) the generation process, which requires time since the emissions of urban, industrial, and biogenic precur-sors to the production of O3, and (ii) the consumption (NO

titration) of O3that takes place in urban areas.

Other studies, such as Sicard et al. (2013), Paoletti et al. (2014), Escudero et al. (2014), García et al. (2014), Querol et al. (2014, 2016), and EMEP (2016), also provided evi-dence that there is a general tendency for O3 to increase

in urban areas, including at traffic sites, probably due to the greater reduction of NO emissions relative to NO2 and

therefore a lower NO titration effect. This trend in decreas-ing NO/NO2ratios from diesel vehicle emissions (the main

source of NOX in urban Europe) has been widely reported

15 years, while acute episodes have been drastically reduced compared to the late 1990s, although these markedly increase during heat waves, such as those in the summers of 2003 and 2015 (EEA, 2017; Diéguez et al., 2009, 2014; Querol et al., 2016).

A recent study (Saiz-Lopez et al., 2017) reported an in-crease of 30–40 % in ambient air O3levels, along with a

de-crease of 20–40 % in NO2, from 2007 to 2014 in Madrid,

which may have led to large concentration increases of up to 70 and 90 % in OH and NO3, respectively, thereby changing

the oxidative capacity of this urban atmosphere. We still do not know if this increase is due to a decrease in the NO titra-tion effect or to the fact that O3formation is dominated by

VOCs since urban areas are characterised by “VOC-limited” conditions, and a reduction in NOxemissions might yield an

increase in O3formation.

Intensive research on O3pollution has been carried out in

the Mediterranean since the late 1980s and has been key in understanding the behaviour of this pollutant in Europe. It has also been used to establish current European air qual-ity standards (Millán et al., 1991, 1996a, b, c, 2000, 2002; Millán, 2002; Lelieveld 2002; EC, 2002, 2004; Millán and Sanz, 1999; Mantilla et al., 1997; Salvador et al., 1997, 1999; Gangoiti et al., 2001; Stein et al., 2004, 2005; Chevalier et al., 2007; Kalabokas et al., 2008, 2015, 2017; Castell et al., 2008a, b, 2012; Kulkarni et al., 2011; Velchev et al., 2011; Doval et al., 2012; Sicard et al., 2013; Millán, 2014; Es-cudero et al., 2014; Zanis et al., 2014; Sicard et al., 2017, among others). The EEA (2017) reports a clear increase in exceedances of the human protection 8 h O3target values in

southern and central Europe, which are higher in the Italian Po Valley and Spain and relatively lower in Portugal and the Eastern Mediterranean.

Focusing on the study area, Diéguez et al. (2009, 2014) de-scribe in detail the temporal and spatial variation in O3levels

in Spain. These studies highlight the low inter-annual vari-ability in regional background stations and the existence of specific areas, such as the Madrid air basin (MAB), north-ern valleys influenced by the Barcelona urban plume, Puer-tollano basin, and the interior of the Valencian region where very high O3episodes are relatively frequent, and point to

ur-ban and industrial hot spots as relevant sources of precursors. Recently, Querol et al. (2016) provided evidence that the highest O3episodes, with hourly exceedances of the

infor-mation threshold for informing the population (180 µg m−3)

during 2000–2015, occurred mostly around these densely populated or industrialised areas.

Querol et al. (2017) report that the high-O3 plume

trans-ported from the metropolitan area of Barcelona contributed decisively to the frequent exceedances of the information threshold in the northern areas of Barcelona during the acute O3episodes in July 2015. They also demonstrate that the

as-sociated meteorology was very complex, similar to the sce-narios involving the vertical recirculation of air masses re-ported by Gangoiti et al. (2001), Millán (2014), and Diéguez

et al. (2014) for other regions of the Western Mediterranean. The regional transport of O3is also very relevant for the

oc-currence of acute O3 episodes causing exceedances of the

information threshold. It is also shown that the vast majority of these exceedances are recorded in July.

In the Eastern Mediterranean, the regional background O3

levels in the free troposphere and the upper boundary layer during summer might regularly exceed 60 ppb, and the fu-migation of these upper air masses contributes on average to the greatest part of the surface O3levels measured in Greece

(Kalabokas et al., 2000; Kourtidis et al., 2002; Kouvarakis et al., 2002; Lelieveld et al., 2002; Kalabokas and Repapis, 2004; Gerasopoulos et al., 2005). Furthermore, a number of studies report contributions from the stratosphere to the sur-face O3 concentrations during specific meteorological

sce-narios in the same region (Kalabokas et al., 2013, 2015; Za-nis et al., 2014; Parrish et al., 2012; Lefohn et al., 2012; Akri-tidis et al., 2016, among others). In addition, recent research shows that during springtime O3episodes (April–May) over

the WMB, similar synoptic meteorological patterns might also occur, and that these are linked with regional episodes, mainly induced by large-scale tropospheric O3 subsidence

influencing the boundary layer and the ground surface O3

concentrations (Kalabokas et al., 2017). However, the most intense episodes in the WMB occur in June–July according to the statistics for the 2000–2015 period in Spain presented by Querol et al. (2016).

In addition to primary emissions, nucleation or new par-ticle formation (NPF) processes give rise to relevant contri-butions to the urban ambient air UFP concentrations, mostly during photochemical pollution episodes in spring and sum-mer (Brines et al., 2015, and references therein). Ambient conditions favouring urban NPF are high insolation, low rela-tive humidity, available SO2and VOCs, and a low

condensa-tion sink potential (i.e. a relatively clean atmosphere with low surface aerosol concentrations; Kulmala et al., 2000, 2004; Kulmala and Kerminen, 2008; Sipilä, et al., 2010; Salma et al., 2016).

In this study, we evaluate the temporal and spatial variabil-ity of O3and UFPs in the MAB (4–20 July 2016) to

investi-gate the causes of acute summer episodes of both pollutants and possible inter-relationships. In a subsequent companion paper (Carnerero et al., 2018), we will focus on the phe-nomenology of UFP nucleation episodes linked with these photochemical events. Data on UFPs are included in this pa-per only where they assist in interpreting the behaviour of O3.

2 Methodology 2.1 The study area

A-A’

_B-B’

CSIC(MADRID)

CSIC CIEMAT ISCIII

MJDH

MJDH-R.C.

city

17 km

N

1000

500

GUADALAJARA CSIC (MAD)

TOLEDO

50 75 150 200 250 3 00 2000

1500

1000

500

25 50 75 100 125 150 170

MJDH CIEMATCSIC (MAD)

A-A’

B-B’

Guadarrama M.R.

Toledo Ms. Iberian R.

El

ev

ati

on

(m

a.s

.l.)

Distance (km) Distance (km)

54 km

Figure 1.Location of the study area, profiles showing the major orographic patterns, location of three supersites (CSIC, CIEMAT, ISCIII), and the site where vertical profile measurements were carried out (MJDH).

the Guadarrama range, which runs in the NE–SW direction, reaches heights of up to 2400 m a.s.l. and is located 40 km north from the MMA. To the S, are the Toledo Mountains, which run from E to W (Fig. 1). Lower mountains, located to the NE and E, are part of the Iberian range. Consequently, the Madrid plain shows a NE–SW channelling of winds forced by the main mountain ranges and following the basin of the Tagus River and its tributaries. In particular, the MMA is lo-cated to the NE of the river basin and on its E side.

Climatologically, the area is characterised by continental conditions with hot summers and cold winters, with both sea-sons typically being dry. Mean annual precipitation of ap-proximately 400 mm is mainly concentrated in the autumn and spring. The MMA is one of the most densely populated regions in Spain, with more than 5 million inhabitants, in-cluding Madrid and the surrounding towns. According to Salvador et al. (2015), anthropogenic emissions are domi-nated by road traffic and residential heating (in winter), with minor contributions from industry and a large airport.

Figure 2 shows the time series of the recorded meteorol-ogy measured at a surface station representative of the con-ditions in the MMA during the field campaign of July 2016 (El Retiro in central Madrid). In order to put the field

cam-paign into the context of the more general meteorological situation, the time series is extended backwards to the end of June and forward to the end of July 2016. Figure 2 also shows the corresponding time series for O3, NO2, and OX

concentrations in the MMA, demonstrating the occurrence of well-marked peaks alternating with relatively low O3and Ox

concentrations periods. The intensive field campaign (11–14 July 2016, marked with a green frame) coincides with a low O3interval preceding a higher O3period in the last 2 days.

Red and blue frames in Fig. 2 show days on which high-resolution O3free soundings were performed (red and blue

indicating intervals within high and low O3, respectively).

2.2 Monitoring sites and instrumentation

To characterise acute summer episodes of O3and UFPs and

to investigate their possible relationships, we devised an in-tensive field campaign in the MMA. Three measurement su-persites in and around Madrid, following a W-NW direction according the previously described dynamics, were deployed in an area where the highest levels of O3(with hourly

El Pardo S. Martín de V. Majadahonda Campisábalos

0 20 40 60 80 100 120 140 160 180 200 28 /0 6 29 /0 6 30 /0 6 0 1 / 0 7 02 /0 7 03 /0 7 04 /0 7 05 /0 7 06 /0 7 07 /0 7 0 8 / 0 7 09 /0 7 10 /0 7 11 /0 7 12 /0 7 13 /0 7 14 /0 7 15 /0 7 16 /0 7 17 /0 7 18 /0 7 19 /0 7 20 /0 7 21 /0 7 22 /0 7 23 /0 7 24 /0 7 25 /0 7 26 /0 7 27 /0 7 28 /0 7 29 /0 7 30 /0 7 31 /0 7 01 /0 8 NO 2 (µ g / m 3)

El Pardo S. Martín de V. Majadahonda Campisábalos

0 10 20 30 40 50 60 70 80 90 100 0 30 60 90 120 150 180 210 240 270 300 330 360 2 8 /06 2 9 /06 3 0 /06 0 1 /07 0 2 /07 0 3 /07 0 4 /07 0 5 /07 0 6 /07 0 7 /07 0 8 /07 0 9 /07 1 0 /07 1 1 /07 1 2 /07 1 3 /07 1 4 /07 1 5 /07 1 6 /07 1 7 /07 1 8 /07 1 9 /07 2 0 /07 2 1 /07 2 2 /07 2 3 /07 2 4 /07 2 5 /07 2 6 /07 2 7 /07 2 8 /07 2 9 /07 3 0 /07 3 1 /07 0 1 /08 R H (%) , T e mp ( ⁰C ) , Win d sp e e d () Win d D ir e ctio n ( ⁰)

Wind Direction RH Wind speed Temp

360 270 180 90 0 WD IR (d e g ) R H (% ), T ( C ), o W V E L (x 1 0 m s -1) O3 (µ g / m 3) NO 2 (µ g / m 3) Profiling campaign 0 50 100 150 200 250 28 /0 6 29 /0 6 3 0 /0 6 01 /0 7 02 /0 7 03 /0 7 04 /0 7 05 /0 7 06 /0 7 07 /0 7 08 /0 7 09 /0 7 10 /0 7 11 /0 7 12 /0 7 13 /0 7 14 /0 7 15 /0 7 16 /0 7 17 /0 7 18 /0 7 19 /0 7 20 /0 7 21 /0 7 22 /0 7 23 /0 7 24 /0 7 25 /0 7 26 /0 7 27 /0 7 28 /0 7 2 9 /0 7 30 /0 7 31 /0 7 01 /0 8 O3 (µg /m 3)

El Pardo San Martin Majadahonda Campisábalos

0 50 100 150 200 250 28 /0 6 29 /0 6 30 /0 6 01 /0 7 02 /0 7 03 /0 7 04 /0 7 05 /0 7 06 /0 7 07 /0 7 08 /0 7 09 /0 7 10 /0 7 11 /0 7 12 /0 7 13 /0 7 14 /0 7 15 /0 7 16 /0 7 17 /0 7 18 /0 7 19 /0 7 20 /0 7 21 /0 7 22 /0 7 23 /0 7 24 /0 7 25 /0 7 26 /0 7 27 /0 7 28 /0 7 29 /0 7 30 /0 7 31 /0 7 01 /0 8 Ox (µ g / m 3)

El Pardo S. Martín de V. Majadahonda Campisábalos

OX (µg/m 3) 0 50 100 150 200 250 28 /0 6 29 /0 6 30 /0 6 0 1 /0 7 02 /0 7 03 /0 7 04 /0 7 05 /0 7 06 /0 7 07 /0 7 08 /0 7 09 /0 7 10 /0 7 1 1 /0 7 12 /0 7 13 /0 7 14 /0 7 15 /0 7 16 /0 7 17 /0 7 18 /0 7 19 /0 7 20 /0 7 21 /0 7 22 /0 7 23 /0 7 24 /0 7 25 /0 7 26 /0 7 27 /0 7 28 /0 7 29 /0 7 30 /0 7 31 /0 7 01 /0 8 O3

El Pardo San Martin de V. Majadahonda Campisábalos

0 50 100 150 200 250 28 /0 6 29 /0 6 30 /0 6 0 1 /0 7 02 /0 7 03 /0 7 04 /0 7 05 /0 7 06 /0 7 07 /0 7 08 /0 7 09 /0 7 10 /0 7 1 1 /0 7 12 /0 7 13 /0 7 14 /0 7 15 /0 7 16 /0 7 17 /0 7 18 /0 7 19 /0 7 20 /0 7 21 /0 7 22 /0 7 23 /0 7 24 /0 7 25 /0 7 26 /0 7 27 /0 7 28 /0 7 29 /0 7 30 /0 7 31 /0 7 01 /0 8 OX

El Pardo San Martin de V. Majadahonda Campisábalos

0 20 40 60 80 100 120 140 160 180 200 28 /0 6 29 /0 6 30 /0 6 01 /0 7 02 /0 7 03 /0 7 0 4 /0 7 05 /0 7 06 /0 7 07 /0 7 08 /0 7 09 /0 7 10 /0 7 11 /0 7 12 /0 7 13 /0 7 1 4 /0 7 15 /0 7 16 /0 7 17 /0 7 18 /0 7 19 /0 7 20 /0 7 21 /0 7 22 /0 7 23 /0 7 24 /0 7 25 /0 7 26 /0 7 2 7 /0 7 28 /0 7 29 /0 7 30 /0 7 31 /0 7 01 /0 8 N O2 (µ g m -3)

El Pardo San Martin de V. Majadahonda Campisábalos

Profiling

campaign El Retiro

(µ g m -3) (µ g m -3)

(a)

(b)

(c)

Figure 2. (a)Hourly meteorological parameters recorded at El Retiro air quality monitoring station in central Madrid (from 28 June 2016 to 1 August 2016).(b)Hourly concentrations of O₃and O_X(O₃+NO₂)recorded at a selection of air quality monitoring stations representing the greater Madrid area, together with those from the remote background station of Campisábalos.(c)Hourly NO2concentrations recorded

at the same sites for the same period. Periods with available AEMET free soundings of O3are bracketed with red (accumulation) or blue

(Reche et al., 2018) inside the MAB (Fig. 1). Table 1 shows the equipment available at the following three supersites:

– Madrid-CSIC, located at the Spanish National Research Council headquarters (this site is located in central Madrid on the sixth floor of the building of the Instituto de Ciencias Agrarias);

– CIEMAT, located at the Centro de Investigaciones En-ergéticas Medioambientales y Tecnológicas headquar-ters, 4 km in the W-NW direction from the CSIC site in a suburban area; and

– MJDH-ISCIII, located in the Instituto de Salud Carlos III in Majadahonda, 15 km in the NW direction from the CSIC site.

At MJDH-ISCIII, a PTR-ToF-MS (proton-transfer reac-tion time-of-flight mass spectrometer) was deployed from 4 to 19 July 2016 and provides insights into the O3formation

potential (OFP) of the VOC mixture over the MMA. The op-eration procedure of the PTR-ToF-MS and OFP calculation are detailed in Table S1 and Fig. S1 in the Supplement.

Furthermore, from 11 to 14 July 2016, 28 profiles of pol-lutant and meteorological parameters up to 1200 m above ground level (m a.g.l.) were obtained using tethered bal-loons and a fast winch system (Fig. S2, Table 2). The in-strumentation attached to the balloons is summarised in Ta-ble 1. The profiles were performed at the Majadahonda rugby course (MJDH-RC Fig. 1). The balloons were equipped with a global positioning system (GPS) and a set of instruments (Fig. S3), including the following.

– A miniaturised CPC (condensation particle counter built by Hanyang University, Hy-CPC) was used to measure the number concentration of particles larger than 3 nm (PN3)with a time resolution of 1 s and a flow rate of

0.125 L min−1using butanol as a working fluid (Lee et al., 2014). Previous inter-comparison studies with con-ventional CPCs have yielded very good results (withr2

reaching 0.65–0.98 and slopes 0.87–1.23; Minguillón et al., 2015; reference and Hy-CPCs had different size de-tection limits.). In this work, we will use the terms UFP and PN3 as equivalents, but we measure concentrations between 3 and 1000 nm strictly, while UFP is < 100 nm. However, 80 % of the total particle concentration falls in the range of UFPs.

– An O3 monitor (PO3M, 2B Technologies) was

used to determine O3 concentrations. It was

cali-brated against an ultraviolet spectrometry reference analyser (RefO3) showing good agreement (n=34;

PO3MO3=1.1058×RefO3+4.41, R2=0.93).

Con-centrations (on 10 s basis) are reported in standard con-ditions (20◦C and 101.3 kPa) and corrected for the ref-erence method.

In addition to the above instrumentation, we obtained the following additional meteorological and air quality data.

– Meteorological data from the CIEMAT meteorologi-cal tower (four instrumented levels between the sur-face and 54 m a.g.l.) and from several AEMET (Span-ish Met Office) standard meteorological stations spread out across the basin were collected: Madrid air-port (40.46◦N, 3.56◦W; 609 m a.s.l.), Colmenar Viejo (40.69◦N, 3.76◦W; 994 m a.s.l.), and El Retiro (in Madrid, 40.40◦_{N, 3.67}◦_{W; 667 m a.s.l.).}

– Hourly data for air pollutants (NO, NO2, SO2, O3,

PM10, and PM2.5)were supplied by the air quality

net-works of the city of Madrid, the regional governments of Madrid, Castilla La Mancha, Castilla y León, and the European Monitoring and Evaluation Programme (EMEP) monitoring network, all of them collected by the National Air Quality Database of the Ministry of the Environment of Spain (MAPAMA).

– High-resolution O3 sounding data were gathered by

AEMET at midday each Wednesday at the Madrid air-port.

– High-resolution meteorological sounding data were obtained each day at 00:00 and 12:00 local time by AEMET, also at the Madrid airport. They were used to estimate the height of the planetary boundary layer (PBL) at 12:00 UTC by means of the simple par-cel method (Pandolfi et al., 2014).

Hourly averaged wind components were calculated and used in polar plots with hourly PM1, PM2.5, NO2, O3,

Ox(O3+NO2), BC, and UFP concentrations by means of the

OpenAir R package (Carslaw and Ropkins, 2012).

3 Results

3.1 Meteorological context

The AEMET O3 soundings are represented in Fig. 3, from

which it is evident that the low and high O3periods coincide

with the 500 hPa gph passage of, respectively, upper-level troughs and ridges over the area associated with the cold or warm deep advection of air masses. Cold advections usually have an Atlantic origin.

The local meteorology during the field campaign was char-acterised by a progressive drop in temperature (T;−4◦_{C in}

Table 1.Details of the instrumentation used at the three supersites and the platform mounted on tethered balloons. BC, black carbon; UFPs, ultrafine particles; CPC, condensation particle counter; OPC optical particle counter; MAAP, multi-angle absorption photometer; PTR-ToF-MS, proton-transfer reaction time-of-flight mass spectrometer.

Site Latitude (N) Longitude (W) Elevation Parameter Operation

(m a.s.l.) (device and model) period

CSIC 40◦2602500 03◦4101700 713 NOx(Teledyne API 200EU)

O₃(2B Technologies 202) UFP > 2.5 nm (CPC-TSI 3775) BC (Aethalometer-AE33) PM1(OPC-GRIMM 1107)

9–20 July 2016

CIEMAT 40◦2702300 03◦4303200 669 NOX(THERMO 17i)

O3(THERMO 49i)

UFP > 7 nm (CPC-TSI 3772) UFP > 2.5 nm (CPC-TSI 3776) BC (Aethalometer-AE33) PM_2.5(TEOM©) Meteorological tower

4–20 July 2016

ISCIII 40◦2702700 03◦5105400 739 NO_X(THERMO 17i)

O3(THERMO 49i)

UFP > 7 nm (CPC-TSI 3783) UFP > 2.5 nm (CPC-TSI 3776) BC (MAAP–THERMO) PM1(OPC-GRIMM 1108)

PTR-ToF-MS (HR 8000, Ioni-con; operating procedures de-scribed in SI)

4–20 July 2016

MJDH-RC (vertical profiles) 40◦2803000 03◦5205500 729 UFP > 3 nm (CPC Hy-CPC) O3(PO3M™2B Technologies)

Meteorology (temperature, rel-ative humidity, pressure, wind speed and direction)

11–14 July 2016

El Retiro 40◦2405500 03◦4100400 667 Meteorological parameters 4–20 July 2016

Colmenar in Fig. S4. This is probably related to drainage (katabatic) conditions inside the MAB, with a progressive turn to a more synoptic westerly component in the central pe-riod of the day, consistent with a convective coupling with the more intense upper-level wind. This coupling is also accom-panied by an important increase in the wind speed at midday, up to 8 m s−1(venting stage), that renewed air masses in the whole basin.

During the second half of the campaign, intense and per-sistent north-easterly winds replaced the westerlies from the evening of 12 July 2016 on, after the evolution of the upper-level trough. In contrast to the previous period, during 13– 14 July 2016, night-time and early morning conditions reg-istered more intense NE winds (up to 10 m s−1_{) than at}

mid-day, after a decrease in intensity down to calm conditions (1 m s−1) during the morning of 12 July, facilitating both fu-migation from upper levels and local O3photochemical

pro-duction. A weak wind veering to the south was also reg-istered at the mentioned surface stations during the

after-noon of 13 July, which lasted for only 3 h and which is more characteristic of an O3 enrichment episode, when the

veering lasted longer (Plaza et al., 1997). A progressive de-crease in the PBL height (−600 m difference) is observed in the AEMET daily radio soundings, in particular gradual decreases in the midday PBL height of 3400, 2200, 1900, and 1600 m a.s.l. from 11 to 14 July 2016 (Fig. S5) were ob-served. This decrease is also observed in the 12 and 14 July 2017 UFP profiles (Figs. 5 and 6 and S6–S8). As will be detailed later, these meteorological patterns allowed O3and

UFPs to smoothly and progressively accumulate in the basin (Fig. 4) during the campaign.

In the vertical dimension during both the high and low O3periods analysed here, all the soundings show at midday

two well-defined layers separated by a temperature inversion marking the limit of the growing convection inside the PBL (Fig. 3).

In high O3periods (6 and 27 July 2016), we found lower

Table 2.Vertical measurement profiles obtained during 11–14 July 2016 at Majadahonda (MJDH-RC).

Day Starting Final Number Maximum

hour (UTC) hour (UTC) of profiles altitude (m a.g.l.)

11 July 2016 18:30 18:45 2 200

12 July 2012 07:02 07:40 2 850

08:30 09:10 2 1000

10:10 10:56 2 1100

11:55 13:43 2 900

13 July 2008 10:45 11:25 2 1000

11:25 12:00 2 1000

13:47 14:29 2 1000

14:29 15:12 2 1100

14 July 2004 08:03 08:44 2 1150

08:48 10:37 2 1100

10:46 12:45 2 1200

13:22 14:02 2 1100

15:23 16:13 2 1025

17:12 17:31 2 1100

winds from the E or NE (less than 4–5 m s−1) or calm condi-tions. This is consistent with the scheme proposed by Plaza et al. (1997), who also describe a rapid evolution of the PBL height up to 2500–3000 m a.s.l. at 15:00 UTC during their field campaigns in the area under “summer anticyclonic con-ditions.” They also describe a morning radiative surface in-version at around 1000 m a.s.l., which was usually “destroyed 1 h after dawn,” containing NE winds associated with noctur-nal drainage flows at lower levels (following the slope of the MAB). In this context, residual layers containing pollutants processed during the previous day(s) can develop above the stably stratified surface layer during night-time conditions. These pollutants can be transported towards the S by weak north-easterly winds or remain stagnant under calm condi-tions, which leads to fumigation and mixing with fresh pollu-tants emitted at the surface after the destabilisation of the sur-face layer, as evidenced in our profiles. These residual layers are topped by the subsidence anticyclonic inversion (1000– 1500 m a.s.l.) according to Plaza et al. (1997).

Conversely, the soundings corresponding to low O3

pe-riods have in common more elevated PBL heights (2000-2500 m a.s.l.), with more intense winds (above 6–7 m s−1) that can blow from different sectors: from the NE on 13 July 2016 (with intense north-westerlies blowing in the free tro-posphere) or the S-SW as observed on 29 June 2016 and 20 July 2016. The O3sounding on 13 July 2016, a unique day

within the field campaign, presents the final stage of a low O3period, with winds in the free troposphere having a clear

NW component, while channelled north-easterly winds dom-inate below 2000 m a.s.l. The AEMET free sounding shows low O3 surface concentrations (< 45 ppb) and high levels

(> 70 ppb) in the middle troposphere (3000–5000 m a.s.l.) as-sociated with very low relative humidity and intense W to

NW winds blowing at that height, which will be discussed in Sect. 4. The decrease in surface temperature observed in Fig. 2 during the field campaign is also consistent with the cold advection associated with the troughing in the 500 hPa heights (13 July 2016 in Fig. 3).

3.2 Surface O3, OX, and UFP during the field

campaign

As previously stated, the field campaign was characterised by atmospheric venting conditions with the two last days mark-ing a transitional period to a more stable anticyclonic episode of increasing O3. The lowering of the wind speed during

di-urnal periods and other meteorological features mentioned above favoured the gradual accumulation of pollutants, as indicated by the progressive increase in the O3 maxima

at MJDH-ISCIII, where the O3 maximum was reached at

15:00 UTC on 13 July 2016 and at 17:00 UTC on 14 July 2016 (Fig. 4). The typical accumulation O3 cycle for the

0 45 90 135 180 225 270 315 360

Altitud e a.s .l. (m) 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0 4 0 0 0 4 5 0 0 5 0 0 0

0 45 90 135 180 225 270 315 360

Altitu d e a.s.l. (m) 5 00 1 00 0 1 50 0 2 00 0 2 50 0 3 00 0 3 50 0 4 00 0 4 50 0 5 00 0

0 45 90 135 180 225 270 315 360

Altitu d e a.s.l. (m) 5 0 0 1 0 0 0 1 50 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0 4 0 0 0 4 5 0 0 5 0 0 0

0 45 90 135 180 225 270 315 360

Altitu d e a.s.l. (m) 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0 4 0 0 0 4 5 0 0 5 0 0 0

0 45 90 135 180 225 270 315 360

Altitu

d

e

a.s.l. (m)

0 90 180 270 360

5000 4000 3000 2000 1000 500 El e vati o n (m a.s.l .)

0 90 180 270 360

5000 4000 3000 2000 1000 500 El e vati o n (m a.s.l .)

0 90 180 270 360

5000 4000 3000 2000 1000 500 El e vati o n (m a.s.l .)

0 90 180 270 360

5000 4000 3000 2000 1000 500 El e vati o n (m a.s.l .)

0 90 180 270 360

5000 4000 3000 2000 1000 500 El e vati o n (m a.s.l .)

T (o_C)

RH (%)

WVEL (x 10 m s-1₎ WDIR (deg) O (x 2 ppb)₃

Dry adiabatic (x 10 C)o

600 580 560 540 520 500 480 Ge o p o ten ti al h ei gh t (m)

0.0 9.0 18.0 27.0 36.0

0.0 9.0 18.0 27.0 36.0

0.0 9.0 18.0 27.0 36.0

0.0 9.0 18.0 27.0 36.0

0.0 9.0 18.0 27.0 36.0

(a)

(b)

Figure 3. (a)Climate Forecast System Reanalysis (CFSR) for the 500 hPa geopotential heights (gpdams) and mean sea level pressure (MSLP) contours (hPa) at 12:00 UTC (obtained from the Climate Forecast System reanalysis; Saha et al., 2014) in July 2016 (Wetterzentrale, http: //www.wetterzentrale.de/; last access: 2 November 2017) shown simultaneously with(b)AEMET O3free soundings at the Madrid airport.

From 11 to 12 July 2016 the highest concentrations of O3

were recorded for W-SW and W winds, and peak UFP (PN3)

concentrations were observed with W, SW, W-NW, and NE winds. However, on 13–14 July 2016, both O3and UFP

con-centrations maximised during calm and NE winds (see polar

plots from Fig. S10). PM2.5 levels were independent of the

UFP and O3variation, with concentrations increasing in calm

Figure 4.Variation in meteorological parameters (temperature, relative humidity, solar radiation, and wind speed and direction) and levels of NO₂, NO, O₃, PM_2.5, PM₁, BC, and UFPs (with lower detection limits of 1, 3, and 7 nm for PN₁, PN₃, and PN7)measured at Madrid-CSIC,

Madrid-CIEMAT, and ISCIII, as well as in MJDH-RC from 11 to 14 July 2016.

3.3 Vertical O3and UFP profiles during the field

campaign

As shown in Fig. S2 and Table 2, the vertical profiles for 14 July 2016 were the most complete of the campaign (wind

speed was relatively low and this allowed for extended mea-surements throughout the day), and for that reason we begin with the description of this day.

14/07/2016 14/07/2016

2000

1600

1200

800 1800

1400

1000

600

2000

1600

1200

800 1800

1400

1000

600

D

A

10 20 30 40 50

0 25 50 75 100

l l llll l l l lllll l l l lllll l l l l llll l l

10 15 20 25 30

l l llll l l l l llll l l l l llll l l l lllll l l

103 ₁₀4 ₁₀5

m

a

.s.

l.

T (o_C)

PN3–300(no. cm )-3

O₃(ppb)

m

a

.s.

l.

RH (%)

35

08:05–08:20 08:50–09:07 09:27–09:35 10:45–11:01 UTC, hh:mm

08:20–08:46 09:07–09:25 10:35–10:43 11:01–11:23 UTC, hh:mm

l

2000

1600

1200

800 1800

1400

1000

600

2000

1600

1200

800 1800

1400

1000

600

D

A

10 20 30 40 50

12:03–12:16

0 25 50 75 100

l l llll l l l lllll l l l lllll l l l l llll l l

10 15 20 25 30

l l llll l l l l llll l l l l llll l l l lllll l l

103 ₁₀4 ₁₀5 ₃₅

m

a

.s.

l.

O3(ppb)

m

a

.s.

l.

RH (%)

13:25–13:48 15:36–15:55 17:13–17:30 UTC, hh:mm

12:16–12:34 13:48–14:02 15:55–16:11 17:30–17:45 UTC, hh:mm

14/07/2016

T (o_C)

PN3–300(no. cm )-3

Figure 5.Vertical profiles of levels of O3, UFP (PN3), temperature, and relative humidity obtained on 14 July 2016 (08:05 to 17:45 UTC).

A: ascending; D: descending.

6522 X. Querol et al.: Summer ozone episodes over the Madrid Metropolitan Area

09:32 09:47 10:02 10:17 10:32

UTC (hh:mm) R H % 80 60 40 O3 ppb 105 104 103 W in d sp e e d m /s 6 4 2 0

11:13 11:28 11:43 11:58

UTC (hh:mm) 10 20 50 100 200 Dp ( nm ) 30 40 70 10 20 50 100 200 Dp ( nm ) 30 40 7– 1 Dp (nm ) 2000 1600 1200 800 1800 1400 1000

600 l l llll l l l l llll l l

l l llll l l l llll l l

103 ₁₀4 ₁₀5

103 ₁₀4 ₁₀5

09:07–09:25 10:45–11:01 12:16 13:48–14:02 15:55–16:11 17:30–17:45 UTC, hh:mm Al ti tu de m a.s. l. PN3-300 l

2

3

4

5

6

7

08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

Time UTC (hh:mm)

18:00 2000 1600 1200 800 1800 1400 1000 600 A lti tu de m a. s. l.

a

b

c

09:32 09:47 10:02 10:17 10:32

UTC (hh:mm) R H % 80 60 40 O3 ppb 105 104 103 W in d sp e e d m /s 6 4 2 0

11:13 11:28 11:43 11:58

UTC (hh:mm) 10 20 50 100 Dp ( nm ) 30 40 70 10 20 50 100 Dp ( nm ) 30 40 70 Dp (nm )

PN

(no. cm )

O3 (p p b ) El e vation (m a.s .l .) PN 3– 30 0 (n o . cm 3-) El e vation (m a.s .l .) El e vation (m a.s .l .) –12:34

Figure 6.UFP (PN₃)concentrations for different vertical profiles obtained on 14 July 2016, as well as O₃and UFPs during two periods focusing on the evaluation of changes produced at a fixed height when reached by the growth of the PBL.

tical profile of UFP (PN3−300) concentrations. At the

be-ginning of the measurements, the upper limit of the PBL was above 1030 m a.s.l., and in 2 h 40 min it lifted 400 m (around 2.5 m min−1_{). In this initial period, the vertical}

pro-file of O3was characterised by a succession of strata of

dif-ferent concentrations, but a clear tendency to increase with height (around 20 ppb of difference between surface level and 1950 m a.s.l. was observed). The discontinuity of the PBL ceiling reflected in the UFP,T, and RH profiles did not seem to affect the O3 profile at all. In other words, we did

not notice accumulation of O3layers in the top of the PBL,

but instead a general increasing trend towards the highest al-titudes reached with the tethered balloons.

Through the course of the day, the profile of concen-trations of UFPs and O3 became homogenous in the

low-est 1200 m a.g.l. (this being the maximum height reached), and a growth of O3 concentrations at all altitudes was

ob-served until 16:11 UTC. This homogenisation and growth of O3concentrations in the PBL, caused by intense mixing by

convection, resulted in an uneven increase through the day with an increase of 43 ppb at the surface and only 10 ppb at 1900 m a.s.l. (Figs. 5 and S6).

Figure 6 shows the results from measurements taken at a fixed height (1400–1200 m a.s.l.) to capture the effect of the growth of the PBL on O3and UFP levels. We started at

ap-proximately 700 m a.g.l. at 09:32 UTC with 60 ppb of O3and

approximately 6000 no. cm−3. At 10:25 UTC, the top of the PBL reached the balloon, as deduced from the sharp increase in UFP concentrations (up to 20 000 no. cm−3_{). Meanwhile,}

O3 concentrations experienced only a slight decrease,

sug-gesting that O3 fluxes are top-down and not bottom-up, as

recorded for UFPs. From 16:11 UTC onwards, a reduction of O3levels at lower heights was observed (−50 ppb at

sur-face levels from 15:55 to 17:45 UTC, while at 1900 m a.s.l. levels remained stable; Figs 5 and S6).

The soundings from 11 to 13 July 2016 again showed a vertical trend characterised by (i) higher O3concentrations at

the highest sounding altitude in the early morning, (ii) an in-crease in O3concentrations as the morning progressed (more

pronounced at low altitudes), and (iii) homogenous O3

con-centration along the entire vertical profile, except in the sur-face layers where the deposition and titration markedly de-creased O3levels reached at midday. Detailed descriptions

of these soundings (Figs. S7 and S8) can be found in the Supplement.

4 Discussion

Plaza et al. (1997) show, for the summer period in the study area, that the development of strong thermal convective

tivity and the influence of the mountain ranges produce char-acteristic mesoscale recirculations. On the other hand Crespí et al. (1995) report, also for summer and the study area, the development of a very deep mixing layer. These authors re-port that the recirculations contribute markedly to the high O3episodes recorded in the region. The arrangement of the

Guadarrama range favours the early heating of its S slopes, which causes a clockwise turning of wind direction, with a NE component during the night, E and S during the early morning and midday, respectively, and SW during the late afternoon, thus defining the north-western sector downwind of the city as the prone area for O3 transport. Night-time

downslope winds inside the basin induce the observed north-easterlies at lower levels. Influenced by these contributions, the barrier effect of the Guadarrama range against the N and NW (Atlantic) winds, as well as the repeated clockwise cir-culation described above, cause the sloshing of the urban plume of Madrid across the basin. Regarding the vertical scale, Plaza et al. (1997) also show that fumigation from high O3-rich layers (injected by upslope winds the

previ-ous day(s) or transported from other areas outside the MAB) could also contribute to the enhancement of the surface O3

concentrations across the basin. This is attributed to the up-ward gradient in concentrations in the lower 1 km of the at-mosphere measured in the early morning and the subsequent mixing across the PBL at midday. On the other hand, Gómez-Moreno et al. (2011) and Brines et al. (2015) report both intensive summer and winter NPF episodes in the western border of Madrid, often simultaneously with the highest O3

episodes.

Considering the free sounding O3profiles in Fig. 3, high

O3concentrations (> 70 ppb) can be observed above the PBL

between 3000 and 5000 m a.s.l., which may be related to the larger-scale transport of pollutants previously uplifted to the mid-troposphere or originated after a stratospheric in-trusion and a subsequent deep subsidence into the middle troposphere, as is probably the case based on the ECMWF ERA-Interim reanalysis data. The transport of high O3 air

masses in the middle troposphere, as for 13 July 2016 in Fig. 3, was also documented by Plaza et al. (1997) over this area in July 1994 during the final phase of a high O3

pe-riod. More recently, Kalabokas et al. (2013, 2015, 2017), Zanis et al. (2014), and Akritidis et al. (2016), among oth-ers, have shown that similar transport processes of enriched O3 layers at high altitude can contribute to increased

sur-face O3 concentrations during the summer in the Eastern

Mediterranean. This transport has been associated with large-scale subsidence within strong northerly winds in the Eastern Mediterranean (Etesian winds), and the affected layers are drier than average and show negative temperature anomalies. Figure S11 shows the ECMWF ERA-Interim reanalysis to-gether with the AEMET O3free soundings at the Madrid

air-port for 13 July 2016. The ridging at the lower troposphere over the Bay of Biscay at the rear of an upper-level trough (left panels) is accompanied by intense NW winds blowing

0 2000 4000 6000

0 20 40 60 80 100

29/06/2016 11:12 h UTC 06/07/2016 10:53 h UTC 13/07/2016 11:03 h UTC 20/07/2016 10:57 h UTC 27/07/2016 11:06 h UTC

12/07/1994 08:15 h UTC

0 30 60 90 120 150

-60 -40 -20 0 20 40 60

8000

6000

4000

2000

0

15/07/1993 05:15 h UTC 09:45 h UTC 12:00 h UTC

ppb

( )

Al

titu

de

m

a.

g.l

.

Temperature

Alt

it

ude

(m

a.

g.l

.)

Alt

it

ude

(m

a.

g.l

.)

O₃(ppb) O3

(a)

(b)

Figure 7. (a)Vertical profiles of O3levels and temperature obtained

on 12 July 1994 (with free sounding) and 15 July 1993 (with teth-ered balloons). Data obtained from Plaza et al. (1997).(b)Vertical profiles of O3levels of the free soundings by AEMET at the Madrid

airport (26.6 km east of MJDH-RC) in June–July 2016.

in the middle and upper troposphere and NE winds at ground level and up to 2000 m (see the radiosonde profile in the same figure). The stratospheric O3intrusion is associated with the

upper-level trough (Sections A-A and B-B in the figure) and a large area of deep subsidence and extremely low relative humidity observed within the NW flows over Madrid and to the north of the Iberian Peninsula and the Bay of Biscay. The high O3concentrations and low relative humidity of the

ERA-Interim profiles over the airport in Madrid (green and red dotted lines in panel g of Fig. S11) are in agreement with the radiosonde observations in the same panel.

The question now is how much of this O3could fumigate

at ground level. According to the radiosonde data, the mix-ing height top was about 2000 m a.s.l. at midday, but could increase to about 3100 m a.s.l. after the projection of the sur-face temperature increase observed during the afternoon at nearby stations. This height reaches the lower part of the O3

-enriched layer originated in the tropopause folding. Thus, a certain impact seems likely. However, the O3concentrations

Thus, according to the O3 soundings and radio

sound-ings analysed above, previous evidence described by Plaza et al. (1997), and the surface air quality measurements pre-sented here, surface O3formation from precursor emissions

within the MMA seems to develop in the core of regional processes and is modulated by large-scale meteorological conditions, distinguishing two types of episodes.

– Accumulation. This occurs in stable, stagnant

condi-tions and the regional accumulation of pollutants (in the sense of Millan et al., 1997, 2000; Gangoiti et al., 2001; Millán, 2014), with high O3reserve strata accumulated

during the previous day(s) in residual layer(s), high O3

background in the free troposphere due to hemispheric transport and/or stratospheric intrusions, and associated with fumigation around midday of the following day. The O3concentrations are high along the whole

atmo-spheric column, but enriched in the lower section by the additional local formation of O3 within the PBL and

transport–recirculation of the urban plume of Madrid around the area. This transport–recirculation is char-acterised by a net transport to the NW-N during day-time after vertical mixing and to the S and SW dur-ing night-time inside the residual layer and decoupled from a more stable nocturnal surface layer. Typically, pollutants accumulate during periods of 2–6 days, re-sulting in well-marked peak and valley concentration periods that affect background, peri-urban, and in-city stations. This is the case for the O3soundings from 29

June 2016 (not shown) and, particularly, 27 July 2016 (Fig. 7) or the measurements with captive and free bal-loons by Plaza et al. (1997) in 1993 and 1994, with very high concentrations of O3in the lower atmospheric

layers, usually forming a bump in the vertical profile of O3 below a height of 2000 m a.s.l., easily

reach-able after daytime convection (Fig. 7). As illustrated for 6 July 2016, OFP (Table S1 and Fig. S1) may be largely dominated by the carbonyls (mostly formalde-hyde and acetaldeformalde-hyde), followed by aromatic com-pounds (benzene, toluene, and C8, C9, and C10 aromat-ics) when considering the VOC pool during the morn-ing traffic peaks. The influence of aromatic VOCs on OFP rapidly decreases, while the influence of biogenic VOCs (mostly isoprene followed by monoterpenes as primary species and methacrolein, methyl-vinyl-ketone, isoprene-derived isomers of unsaturated hydroxy hy-droperoxides (ISOPOOH), and methylglyoxal as the main secondary species) increases through the day, re-sulting in a similar potential influence of biogenic and aromatic VOCs on O3 formation during accumulation

periods, but with an OFP still dominated by carbonyls.

– Venting. This occurs in advective atmospheric

condi-tions (in the sense of Millan et al., 1997, 2000; Gangoiti et al., 2001; Millán, 2014), with O3 soundings

charac-terised by (probably external) contributions from

high-altitude O3strata and background and their fumigation

on the surface (episodes 11–14 July 2016). There is no accumulation of pollutants above the stable nocturnal boundary layer because more intense and steady winds swept out the local production during the preceding day. OFP contributions of carbonyls (dominating OFP) and aromatic and biogenic VOCs did not significantly vary for 13 and 14 July 2016 from what is described above for 6 July 2016.

As detailed in Sect. 3.1 and 3.2, with the weakening of general atmospheric circulation by the end of the campaign period, O3 and UFPs smoothly and progressively

accumu-lated in the basin (Fig. 5). An observed decrease in the PBL depth (up to−1800 m at midday according to the AEMET radio soundings during the campaign; see Fig. S5) probably also contributed to the progressive increase in pollutant con-centrations through the campaign.

With respect to the vertical variability, the general pattern for UFPs (N3)clearly showed a rapid and marked growth

of the PBL in the first hours of daylight (Fig. 8). In these early stages of the day, O3profiles were characterised by a

succession of strata of different concentrations, but a clear increasing trend towards the higher levels (Fig. 8). The dis-continuity of the PBL ceiling, reflected in the UFP, tempera-ture, and humidity profiles, was not identified as such in the O3profiles (Figs. 5, 6, and S6 to S8). As the day progresses,

the UFP and O3concentration profiles are homogenised and

a progressive diurnal growth of O3concentrations occurs

un-til 16:00 or 17:00 UTC (Fig. 8), most clearly observed at the surface. This vertical variability points to different aspects, such as (i) the relevance of fumigation from high-altitude O3-rich strata; (ii) surface titration by NO and deposition of

O3; (iii) surface photochemical generation of O3 from

pre-cursors (with higher concentrations close to the surface); and (iv) horizontal O3and precursor surface transport from the

urban plume of Madrid towards MJDH-RC. The upper O3

-rich strata might have an external (to the Madrid basin or the Iberian peninsula) origin or might have been injected re-gionally at high altitudes on the previous day(s) by the com-plex recirculations of air masses already reported by Mil-lán et al. (1997, 2000, 2002), EC (2002, 2004), Gangoiti et al. (2001), Mantilla et al. (1997), Castell et al. (2008a, b), and Millán (2014) for the WMB, by McKendry and Lundgren (2000) for other parts of the world, and by Plaza et al. (1997) and Diéguez et al. (2007, 2014) for the Madrid area.

According to the last referenced authors, due to the ori-entation of the Sierra de Guadarrama (Fig. 1), the heating of its S slopes throughout the day forces the wind direc-tion to veer, describing an arc that sweeps the zones to the N of Madrid clockwise from the W to the NE. Dieguéz et al. (2014) show that the O3maxima are recorded at an

12/07/2016 12:55–13:31 UTC

13/07/2016 13:47–14:07 UTC

14/07/2016 13:25–13:48 UTC

11/07/2016 18:28–18:36 UTC

14/07/2016 17:30–17:45 UTC

12/07/2016 08:31–08:54 UTC

14/07/2016 08:05–08:20UTC

12/07/2016 10:12–10:31 UTC

13/07/2016 10:45–11:03 UTC

14/07/2016 10:45–11:01 UTC

25 50 75 100

O3(ppb)

25 50 75 100 25 50 75 100 25 50 75 100

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1600

1200

800 1800

1400

1000

6 00

l l l lll l l l l llll l l l l llll l l l l l lll l l

103 ₁₀4 ₁₀5

l l l lll l l l l llll l l l l l lll l l l l l lll l l

104 ₁₀5

l l l lll l l l l llll l l l l l lll l l l l l lll l l

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PN3–300(no. cm )-3

Ele

va

tion

(m

a.

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l.)

Ele

va

tion

(m

a.

s.

l.)

Figure 8.Profiles of O3and UFPs (PN3)from 11–14 July 2016 grouped by hourly stretches from morning to afternoon.

of photochemical processes on its route from the metropoli-tan area to tens of kilometres away. In addition, our results and those of Plaza et al. (1997) show that O3 fumigation

from high atmospheric layers decisively contributes to the increases in the surface levels, since surface concentrations during our measurements never exceeded those recorded at the highest altitude reached, and at midday homogeneous O3

levels are measured across the lower 1.2 km of the PBL. During the whole month of July 2016, there was a clear veering of the urban plume from Madrid, with night plume transport towards the SW (MJDH-San Martin de V., Figs. 9 and S12) and towards the NW, N-NE, and, in some cases, E-SE during the morning and midday, followed by the decoupling and onset of the evening and nocturnal flow towards the SW. This veering seems to be causally associ-ated with the high O3levels recorded in the W to E areas

surrounding northern Madrid, since the peak concentrations recorded by the official air quality network follow this spa-tial and temporal evolution (Fig. S12) for the exceedances of

the O3information threshold. These plume impacts occur in

periods when the O3 concentration is already high because

of accumulation from one day to the next in the (same) air mass, which is not completely renewed due to general cir-culation conditions. The relevance of the latter has been re-cently demonstrated by Otero et al. (2016), who report maxi-mum temperature as the parameter more directly related to high O3 concentrations in central Europe, whereas in the

WMB region, the O3concentrations were more related to the

concentrations recorded the day before.

On the other hand, the differential afternoon–evening de-crease in O3 surface concentrations, compared with those

found at the top of the soundings, again demonstrates the relevance of high-altitude layers, high O3tropospheric

back-ground, and their fumigation to the surface in the hours of maximum convection.

Regarding the concentrations of UFPs, they were very ho-mogeneous throughout the PBL during the vertical profiles, especially in the hours of maximum convection, showing a

0 20 40 60 80 100 120 140 160

00 06 12 18 00 06 12 18 00 06 12 18 00 06 12 18 00

O3

µ

g

m

-3

20 km

N Campisábalos

El Atazar

Alcobendas Colmenar V. Collado V.

San Martín

Fuenlabrada MJDH

El Pardo

CSIC

CIEMAT _{Major Madrid’s}

ring road M-40

0 20 40 60 80 100 120 140 160 180 200

00 06 12 18 00 06 12 18 00 06 12 18 00 06 12 18 00

OX

µ

g

m

-3

Campisábalos El Pardo El Atazar

Colmenar V Alcobendas Fuenlabrada

MJDH

11/07/2016 12/07/2016 13/07/2016 14/07/2016

11/07/2016 12/07/2016 13/07/2016 14/07/2016

Figure 9.Time evolution of hourly OX(O3+NO2)and O3concentrations from 11 to 14 July 2016 at selected air quality monitoring sites

in the Madrid basin and an external reference site (Campisábalos), as well as the locations of these monitoring sites.

marked increase from 11 to 14 July 2016 for the whole depth for all profiles (Fig. 8). Thus, on 12 July 2016, the upper limit of the PBL (marked by a sharp reduction in UFP levels) reached 900 and 1200 m a.g.l., respectively, in the soundings conducted at 08:05 and 10:12 UTC (Fig. 8). In turn, on 14 July 2016, the top of the PBL exceeded 1200 m a.g.l. only in the afternoon, being constrained to 300 to 700 m a.g.l. from 08:05 and 10:45 UTC (also shown in the progressive loss of

−1800 m in the midday PBL height from 11 to 14 July 2016, as revealed by AEMET radio soundings).

The enhanced convection on 12 July 2016 probably favoured the dilution of UFP concentrations and reinforced the fumigation of O3from the upper levels. Conversely, the

lower development of the PBL on 14 July 2016, causing less surface UPF dilution and lower top-down contributions to O3

surface concentrations, accounted for the opposite O3 and

UFP profiles. Thus, a weaker development of the PBL might result in the increase in UFP concentrations, even if UFP emission–formation rates did not vary significantly. How-ever, we cannot discard the possibility that this UFP increase on the last day was the result of a higher intensity and dura-tion of the nucleadura-tion episodes.

Consideration of the evolution of surface O3

concentra-tions on 11 and 12 July 2016 (as shown in Fig. 9) de-picts a double wave: the first peak around midday (11:00– 14:00 UTC on the first day and 12:00–13:00 on the second) and the second peak in the afternoon–evening (19:00–22:00 and 16:00–20:00 UTC, respectively), showing relative peaks (sometimes just a plateau). We interpret the morning increase in O3concentrations to be dominated by both local

produc-tion and anthropogenic VOCs (Fig. S1), as well as the fumi-gation of upper levels, with an early maximum when layers above are rich in O3 that progressively decreases with

di-lution with surface concentrations. The secondary evening concentration peak corresponds to the advection of a locally enriched O3 air mass (titration always causes O3depletion

towards nocturnal values). When both processes (morning fumigation and evening advection) are not so strong, O3local

production results in a more “typical” diurnal time evolution, with a single maximum at 15:00-16:00 UTC, as seen on 13– 14 July 2016 (Fig. 9).

The relative importance of the local contribution of the MMA to the Ox concentrations registered in the monitoring

stations has also been evaluated by comparing the observa-tions at upwind and downwind locaobserva-tions relative to the city. In this respect, Atazar and Alcobendas (Fig. 9) are located downwind for 11 and 12 July 2016, and MJDH and Fuen-labrada are upwind, while the opposite occurs for 13 and 14 July 2016. As the urban air mass is transported towards the E and NE during the first 2 days, a local Ox contribution is

superimposed on the background at Atazar and Alcobendas, where recorded Oxwas the highest in the basin (Fig. 9). The

contrary holds during the next 2 days, when these sites show lower concentrations than the rest. MJDH and Fuenlabrada

show a reversed behaviour, with lower concentrations during the first 2 days and higher for the last days.

In addition of the local O3, the background contribution

can also be very relevant. At high elevation, changes in the background tropospheric O3 can be attributed to (i)

hemi-spheric background concentrations, (ii) exchange between the free troposphere and boundary layer, and (iii) strato-spheric inputs (Chevalier et al., 2007; Kulkarni et al., 2011; Parrish et al., 2012; Lefohn et al., 2012, 2014; Kalabokas et al., 2013, 2015, 2017; Zanis et al., 2014; Akritidis et al., 2016; Sicard et al., 2017).

5 Conclusions

The phenomenology of O3 episodes in the Madrid

Metropolitan Area (MMA, central Iberia) has been charac-terised. We found that O3 episodes linked with precursors

emitted in the Madrid conurbation are modulated by the com-plex regional atmospheric dynamics.

Vertical profiles (up to 1200 m a.g.l.) obtained using teth-ered balloons and miniaturised instrumentation at Majada-honda (MJDH), a suburban site located on the south-western flank of the Madrid Metropolitan Area (MMA) during 11– 14 July 2016, showed how complex O3is with altitude and

time. Simultaneously, measurements of air quality and me-teorological parameters were carried out at three supersites within the MMA, where spatial differences highlight the in-fluence of atmospheric dynamics on different scales.

The results presented here confirm prior findings regard-ing the concatenation of relatively low (ventregard-ing) and high (accumulation) O3 episodes in summer. In the Madrid air

basin (MAB) during both types of episodes, the fumigation of high-altitude O3-rich layers (from a remote or regional

ori-gin) contributes a relevant fraction to surface O3

concentra-tions. Moreover, we propose here a conceptual model (shown in Fig. 10). The following is a more specific description.

– Accumulation episodes are activated by a relatively thinner PBL (< 1500 m a.g.l. at midday), light synop-tic winds, and the development of anabasynop-tic winds along the slope of the Sierra de Guadarrama (W and NW of the MAB, with > 2400 m a.g.l. peaks). This PBL height, lower than the mountain range, and the development of the mountain breezes cause the vertical recirculation of air masses, the enrichment of O3 in the lower

tropo-sphere, and the formation of reservoir layers that fumi-gate to the surface as the diurnal convective circulation develops. This dynamic accounts for the occurrence of the high Ox(O3+NO2)surface concentrations.

– During venting episodes with more intense synop-tic winds and the top of the PBL usually reach-ing > 2000 m a.g.l., vertical O3 profiles were